Improve population raster imports

[r-akemii]

There are two different tasks here:

• Allow the user to use his/hers own population raster files, so if the user has updated files from one country he/she can use it instead of using data from our sources.
• If the user wants to use one of the existing World Pop or Meta files, allow the download of multiple files at the same time. For bigger countries whose population raster files are quite large, it could be really time-consuming waiting for each one of the files to be downloaded.